"It's been 10 years since the Martain's Pandora Box caused the Skywall tragedy. Our country has split into three: Touto, Seito and Hokuto, and the chaos has ensued." ―Narrator
10 years ago, the “Pandora Box” was found on Mars. On Earth, it created walls that divided Japan into Touto (Eastern Capital), Hokuto (Northern Capital) and Seito (Western Capital) and brought along the Smash, unidentified lifeforms that attack people.
Sento Kiryu, prodigy physicist, transforms into Kamen Rider Build to fight the Smash.
The Touto government requests his help in deciphering the mystery behind the Pandora Box and the Smash. There is one mystery not even Sento can solve: his own past. The only memory he has left of more than 20 years is the existence of a sinister figure called, Night Rogue. There is one man who holds the key to Sento’s memories. A prison escapee by the name of Ryuga Banjo, who adamantly denies committing the murder he is accused of. Misora Isurugi who works with Sento, has the ability to purify the harmful components the Smash are made of.
Sento transforms into Kamen Rider Build in order to uncover the mystery surrounding the Pandora Box, the Smash and his amnesia.